1. Field of the Invention
The invention relates to an antenna structure which has two resonant frequency bands or which may be used as the antenna of a radio set in two frequency ranges.
2. Description of Related Art
In different parts of the world cellular telephone systems are in operation with operating frequency ranges which differ significantly one from another. Among the digital cellular telephone systems, the operating frequencies of the GSM (Global System for Mobile Telecommunications) system are in the 890-960 MHz band, those of JDC (Japanese Digital Cellular) 800 and 1500 MHz band, those of the PCN (Personal Communication Network) are in the 1710-1880 MHZ band and those of the PCS (Personal Communication System) in the 1850-1990 MHz band. The operating frequencies of the American AMPS mobile telephone system are 824-894 MHz and the operating frequencies of the DECT (Digital European Cordless Telephone) system are 1880-1900 MHz.
In the mobile telephones designed for these systems, use is generally made of simple cylindrical coil or helical antennae or whip antennae formed from a straight conductor on account of their low manufacturing costs and their relatively good performance. The resonant frequency of an antenna is determined by its electrical length, which should be a specific part of the wavelength of the radio frequency used. The electrical length of a helical antenna used at mobile telephone frequencies should preferably be, for example, 3.lambda./8, 5.lambda./8 or .lambda./4, where .lambda. is the wavelength in use. Similarly, the electrical length of a whip antenna should preferably be, for example, .lambda./2, 5.lambda./8, 3.lambda./8 or .lambda./4. Solutions are also known where the whip- or helical element may be connected in turn to the antenna port of the radio set, and whip-helix series connections which may be pushed partially inside the telephone, for example, as described in International Patent No. WO-92/16980.00. Technical solutions generally involve an attempt to ensure that the antenna is as small as possible during storage and transport, but it may be necessary to pull the antenna out to its external position in order to obtain a better link.
Since the resonant frequency of the antenna according to the prior art is, as has been shown, related to the length of the antenna via the wavelength, it is only possible to use a certain antenna in a mobile telephone that is designed for a cellular telephone system with a single frequency range. In some cases, however, one may wish to use the same telephone in some second frequency range. Then an effective antenna solution is required in addition to the appropriate RF components.
The easiest solution would be to provide the telephone with at least two separate antennae, from which the user can always select for his telephone the antenna which corresponds to the frequency range of the system in use at any time. It has to be assumed, however, that the necessary alternative antenna is generally missing. Continual exchange of the antenna also overtaxes the antenna connector and may over time cause contact disturbances. The second option would be to manufacture at least two fixed antennae of differing dimensions for different points of the telephone, in which case the user would select an antenna by switching into operation the one which corresponded to the frequency range of the system in use. This would add to the number of telephone components and thus increase the manufacturing costs.
U.S. Pat. No. 4,442,438 presents an antenna structure resonating at two frequencies, which essentially consists of two helices HX1, HX2 and one whip element P1, as shown in FIG. 1. The helices HX1 and HX2 are positioned in succession parallel with the axis of symmetry of the structure and their adjacent ends A1 and A2 form the feed point of the combined structure. The whip element P1 lies partially inside the upper helix HX1, projecting to some extent beyond this and its feed point A3 is at the bottom end. The RF signal is carried to the feed point in question A3 via the coaxial conductor KX which lies along the axis of symmetry of the structure and goes through the lower helix HX2. The feed point A3 of the whip element is joined to the lower end A1 of the upper helix and the lower helix is joined at its upper end A2 to the conductive and earthed mantle of the coaxial conductor KX. The first resonating frequency of the structure is the resonating frequency of the combined structure formed by helices HX1 and HX2, which in the embodiment given as an example is 827 MHz. The second resonating frequency of the structure is the common resonating frequency of upper helix HX1 and whip element P1, which in the embodiment in the example is 850 MHz. The helix HX1 and the whip element P1 are thus so designed that they have essentially the same resonating frequency.
The structure presented in this patent is relatively complex and its physical length in the direction of the axis of symmetry is the sum of the physical lengths of the lower helix HX2 and the whip element P1. The greatest drawback of the structure with regard to manufacturing technology is the feed point arrangement at the midpoint of the antenna, where the lower end A3 of the whip element and the lower end A1 of the upper helix have to be in galvanic connection and the lower helix has to be joined at its upper end A2 to the mantle of the coaxial conductor which feeds the whip element. The difference between the two resonating frequencies which are to be attained by the structure is, according to the material presented in the patent, small, since the upper helix H1 and the whip element P1 have to be so dimensioned that they have essentially the same common resonating frequency, so that this antenna cannot for example be used for a telephone operating at GSM and PCN frequencies.